Mobile computing devices have been widely adopted in recent years. Many functions previously performed primarily by personal computers, such as web browsing, streaming, and uploading/downloading of media are now commonly performed on mobile devices. Consumers continue to demand smaller, lighter devices with increased computing power and faster data rates to accomplish these tasks.
Many mobile devices include multiple antennas to provide data rates that satisfy consumers' ever-increasing requirements for upload and download speeds. Integrating multiple antennas into a small form factor device such as a mobile phone or tablet creates the possibility of electromagnetic coupling between antennas. Such electromagnetic coupling has many disadvantages. For example, system efficiency is reduced because signal energy radiated from one antenna is received by another device antenna instead of being radiated toward an intended target. Coupling between antennas becomes even more problematic when the antennas operate at the same or similar frequencies.
Antenna isolation has been attempted through several approaches. One approach is to place antennas sufficiently far apart (e.g., 0.4-0.5 wavelengths) that significant coupling does not occur. Such distances between antennas, however, are not achievable in small form factor devices, especially at lower frequencies. For example, at 700 MHz, antennas would need to be separated by 200 mm (20 cm). Another approach is to create a feedback mechanism that decouples by negating the imaginary part of the mutual impedance. This approach, however, is narrowband and cannot be used for UMTS-like antennas.
Phase-shifting decoupling networks have also been attempted. Because a transmitted signal is known, an out-of-phase version of the transmitted signal can be fed to other antennas to which the transmitted signal is electromagnetically coupled. This creates destructive interference that decouples the antennas. Conventional decoupling networks, however, operate at a single frequency and can also be subject to significant insertion loss that will affect antenna performance.
Orthogonal polarizations of chassis modes have also been attempted with limited success. In this approach, similar antennas (e.g. monopoles) are placed orthogonally on the PCB chassis of a device. Isolation improvement, however, is typically limited to around 3-5 dB, and the device chassis must be large enough to accommodate the orthogonal antennas.